A team of scientists in the United States has found that some plants give out ultrasonic beeps which sound like 'chirps' when they are under stress due to drought or disease. These chirps increase as stress increases and it appears that these chirps are audible to some insects, like the bark beetle which attack ailing trees.
The scientists subsequently developed a method of duplicating the chirps and transmitting them through plant tissue. Then they offered the beetles a variation in diet – plants that chirped and others that did not. The beetles preferred the 'chirping' plants to the silent ones!

This is a very interesting site. Similar to Reagan's question, it would be helpful to have references for this article or name the tree species (as you have in other articles). I would also like information about the scientists who did this work. Looking forward to your reply.